(a) Field of the Invention
The present invention relates to a light emitting display device. More specifically, the present invention relates to a light emitting display device using organic electroluminescent (EL) display device and a driving method thereof.
(b) Description of the Related Art
In general, an active matrix type image display apparatus has a plurality of pixels in the matrix form and controls intensity of light for each pixel according to given brightness information so as to display an image. As for an image display apparatus using liquid crystals as an electro-optic material, the transmittance of each pixel is variable depending on the voltage recorded in the pixel. The active matrix type image display apparatus using an organic EL material as an electro-optic material has the same basic operation as the liquid crystal display devices. Unlike the liquid crystal display devices, however, the organic EL image display apparatus is a self-luminous type that has a light-emitting element such as an Organic Light-Emitting Diode (OLED) in each pixel and exhibits high visibility of images and high response speed without a need for backlights. The brightness of each light-emitting element is controlled by the amount of current. By way of example, the organic EL image display apparatus has a striking difference from the liquid crystal display devices in that the light-emitting element is of a current-driven or current-controlled type.
Methods for driving the organic emission cells are classified into a passive matrix method, and an active matrix method using thin film transistors (TFTs). In the passive matrix method, anodes and cathodes are arranged to cross (i.e., cross over or intersect with) each other, and lines are selected to drive the organic emission cells. On the other hand, in the active matrix method, TFTs are coupled to ITO pixel electrodes, and each organic emission cell is driven according to a voltage maintained by a capacitor coupled to a gate of a TFT. The active matrix method is categorized, depending on the form of a signal applied to the capacitor for establishing the voltage, as a voltage programming method or a current programming method.
The pixel circuit of the conventional voltage programming method has difficulties in obtaining a high gray scale because of deviation of the threshold voltage VTH and the carrier mobility, the deviation being caused by non-uniformity of a manufacturing process. For example, in order to represent 8-bit (i.e., 256) gray scale in the case of driving a TFT by a voltage in the range of 3V (volts), it is required to apply the voltage to the gate of the TFT with an interval of less than 12 mV (=3V/256), but if the deviation of the threshold voltage of the thin film transistor caused by the non-uniformity of the manufacturing process is 100 mV, for example, it is difficult to represent the high gray scale.
The pixel circuit of the current programming method achieves uniform display characteristics even though the driving transistor in each pixel has nonuniform voltage-current characteristics, provided that a current source for supplying the current to the pixel circuit is uniform throughout the whole panel.
The current programming method has a benefit of compensating for the deviation of the threshold voltage and the mobility of the transistor used within the pixels, but it takes a long time to drive a data line with a current of the same magnitude as that of the current that flows to the OLED and this places certain limits to realizing a light emitting display device which has a high gray scale and high resolution.
FIG. 1 shows a configuration of a pixel circuit in a light emitting display device, which uses a current mirror for solving the above-described problem.
As shown, the pixel is formed at a point where a scan line crosses a data line. A signal Scan for selecting a pixel is applied to the scan line according to a predetermined cycle, and brightness information for driving the pixel is applied as a current Idata to the data line.
The pixel includes an OLED 1, two transistors 2 and 3 for configuring a current mirror, a storage capacitor 4 for storing the brightness information converted into a voltage level from the current Idata, and switches 5 and 6 for respectively controlling supply of the current Idata to the transistor 2 and the storage capacitor 4. The pixel circuit of FIG. 1 is coupled to a power line 7 and a ground line 8.
In order to select a pixel, the signal Scan transmitted through the scan line turns on the two switches 5 and 6. In detail, when the switch 5 is turned on, the current Idata including the brightness information applied to the data line flows to the transistor 2, and when the switch 6 is turned on, a voltage corresponding to the current Idata is charged in the storage capacitor 4. When the scan line becomes a non-selection state, the switches 5 and 6 are turned off, and the voltage programmed in the storage capacitor 4 is maintained. As a result, the voltage maintained by the storage capacitor 4 is applied to a gate of the transistor 3, and a corresponding drain current is generated through the transistor 3, thereby driving the OLED 1.
However, in a light emitting display device of the conventional pixel using a current mirror, the brightness is reduced as the location of the pixel becomes farther away from the scan driver.
In further detail, resistance of the switches 5 and 6 is gradually increased and almost no current flows to thus become a turned-off state during a short period in which the pixel is selected and is deselected by the scan line, and the voltage stored in the storage capacitor 4 is maintained. However, since a signal delay is generated because of parasitic elements (e.g., capacitance) of the scan line, and the rising time of the scan signal increases as the pixel is located farther from the scan driver. Therefore, it takes a long time to turn off the switches 5 and 6 in pixels that are located far from the scan driver. In this instance, when the resistance of the switch 5 becomes greater, the voltage at the drain of the transistor 2, i.e., the gate voltage, is increased, and accordingly, a voltage difference between the gate voltage of the transistor 2 and the gate voltage of the transistor 3 is generated. When the rising time of the scan signal is increased in this state, the voltage charged in the capacitor 4 is discharged through the switch 6 and the gate voltage at the transistor is increased since the switch 6 has been insufficiently turned off. Therefore, the brightness is reduced at the pixel which is far from the scan driver. As a result, the brightness over the whole screen does not become uniform and display characteristics are degraded.